The goal of this project is to derive and study a number of models of the countercurrent concentrating mechanism of the renal medulla. In preliminary studies on some simplified models the basic mathematical and computational techniques have been developed. The aim is to develop a model that includes vasa recta, the loop of Henle, and collecting ducts and which includes movement of water, a non-transported solute and a solute that is transported across the wall of the thick portion of the ascending limb of Henle and the wall of the collecting tubules. The model will be checked by simulating a number of experimental procedures and will then be programmed in a tutorial mode for use in teaching at the graduate level and as an aid to research on renal concentrating mechanisms.